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Originally Posted by patch notes
Previously Haste and Slow effects worked inconsistently, with spells working differently from weapons, and hastes and slows not acting as inverses of each other. We have revised the system so that all haste and slow effects work the same way, and haste and slow percentages of the same magnitude perfectly cancel each other out (30% haste and 30% slow combine to no change). As a result, we had to change the tooltip numbers on all spell haste effects, and on all melee and range slow effects. The numbers in the tooltips are different, but the game functionality is unchanged (other than slight rounding errors). Those tooltips that changed will now display larger numbers than they used to display.
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It's not that they were all additive before in this case, as implied in the patch notes. Back in the golden years of 1.11, 30% haste and a 30% slow wouldn't cancel out properly, and a 30% slow on melee and a 30% slow on spell casting wouldn't produce the same decrease. So, Blizzard made some new and improved formulas that fix that issue.
These crazy new formulas require larger magnitudes to achieve the same apparent effect for everything but spell slow effects. In the case of with thunderclap and thunderfury, their effects will be lessened because they have not been increased to compensate for the new formulas.
Not sure what the new formula is exactly, but it should be consistent with the old spell slow formula.